1) High groundwater, due to heavy rains or regional flooding. If area streams and drainage ditches are running full, see if it looks like it might be groundwater backing up and coming to the surface at a low spot in your lawn. Obviously, if immediately adjacent lower areas are not flooded, then not the problem.
2) Drainage from a neighbor's yard, either rain and snowmelt runoff, or a broken water or sewer pipe in their yard. If so, it should be easy to track the wet spot back to their yard - it should lead as a mushy or visibly wet zone from their yard.
3) Leakage from an outside valve leaking through a hose, or a hose left on with leaking spray nozzle (if wet spot is near the end of a hose left out over winter), or an outside valve that is leaking or was left connected to a hose or unprotected so it froze and broke over the winter. Check around all outside piping and hoses for signs of leak.
4) Leakage from your crawl space, garage or unoccupied basement (if you have one) from a broken pipe, that is running out of the house or through/under the foundation. Check all around your foundation and in the basement/crawl space - it is common for such a leak to pool inside the foundation wall, then drain out several feet underground along the loose fill in a ditch where a utility was buried when the house was built, then pop up some distance away over the utility line.
5) Sump pump discharge
6) Illegal outside drain pipe from a hot tub or sauna
7) Illegal outside drain pipe from clothes washer and/or sink
8) Swimming pool pump system leak (especially if it froze with water in it over the winter), or leaking above or in-ground swimming pool liner leak
9) Leakage from an illegally drained water softener/conditioner system, where they ran a drain hose through the wall and outside rather than to the sewer (this is pretty common)
10) Roof gutter drainage, including possibility of a buried extension hose or pipe intended to lead the water further from the foundation
11) Air conditioner condensate drain hose discharge (commonly only a couple of feet long, and does not turn out large quantities of water).
12) Broken pipe or sprinkler head on sprinkler system or running weep watering system - obviously, wet spot should get much more visible in a few minutes if you turn the sprinkler system or weep watering hose on.
13) Overflow from well (due to extreme rainfall raisingthe water table) or from well water system leak (pipes, tanks, etc)
14) sewer backup, resulting in overflow from cleanout (about 4 inch black plastic or cast iron pipe vertical pipe, commonly located about 2-4 feet outside the foundation wall, and may stick up a few inches or may be overgrown and a few inches below ground level).
15) If you have a septic tank and/or leach field, a blockage in the leach field pipes or overfull septic tank will leak to the surface, usually without causing a sewer backup inthe house. Initially appears as a nice green spot over the septic tank of field, then as a surface wet spot, then eventually (over a few days to weeks time) to a bog. Initially, especially if coming up through sandy soil or lawn, it may not stink unless you get your nose right down into it. After it starts pooling on the surface then will definitely stink. If that is the cause, you need a septic system servicing - pumping first to empty the septic tank, and if the problem was not because it was full to the top, then pumping/jetting from inside the tank (a lovely job, I can assure you) along the length of the leach field pipes.
16) Flow into your yard from an undetected drainage pipe. For instance, a house we had in western New York had a low back yard that pooled and swamped in the spring and summer but we didn't want to kill the trees by burying the roots in a couple of feet of fill, so I dug from the back yard to the front yard and buried a drain pipe which lead to a buried cleanout box and french drain located in a property line swale in the front yard - there was no sign of it in the lawn, but when flowing it made a wet zone along the swale as the water drained through the lawn toward the street.
17) Sewer or water pipe break. If the pool is spreading fairly rapidly or forming a rivulet, most likely water service. If only very gradually spreading or not at all, could be either. If you take a stethoscope (or make a 1-2' long cardboard cone or use a plastic traffic cone to listen to the ground, like an antique hearing "horn") you should be able to hear it hissing or moaning or whistling if it is a water service leak (making sure first that there is no water running in the house, of course). If your grass is greening up yet, a water leak will green the grass up a little faster, but except in very dry areas will not promote extreme growth. Sewage leakage will promote very rapid grass growth - wetted area would look like places in a cut lawn do in the spring where dogs have been peeing on it - spots of much faster lush green growth from the added nitrogen. If a water leak and you are on a meter with a meter box by the street, you may have a shutoff valve you can close to see if that stops the flow - otherwise you have to pay your water utility to shut your water off at the streetside "key box", then turn it back on after repair.
@Mixerr. You know I'm only ever trying to help you out, right? That's why I really think a diss video towards
@gimmefreemusic is necessary. That dude is a bully and talks way to much shit about you.