Since the return is still submerged, I would be more inclined to think the skimmer area is the problem. If it's not dropping any further, that should rule-out the plumbing line which is good. I wonder if our recent cold spells effected your plastic or the seal? Have you tried dropping any food color/dye in their with the water quiet to look for areas of escape? Can't really tell from the pic if you have a problem around the skimmer opening (caulking/seal) or if it's the plastic skimmer itself inside. In any case, once you find the gap/crack and location, there are a variety of products that can help in that area such as polyurethane caulk, pool putty like this:
https://www.amazon.com/Atlas-Epoxy-Pool-Putty-Set/dp/B0002Z0NBY, A&B Putty, or perhaps even a 3M 5200 product which is made for use under the water - although that one (and sometimes other products), might require the water level to be dropped slightly for the area to be dry until it cures. So be aware of that as well. If you do need to drop a little water and have a main drain, you can pull from there (backwash to waste). If no main drain, then drop a small sump pump of some type (or siphon) to get clean/dry access to the entire skimmer area if needed to do the repair.
While you do some more thinking and watch that water level for changes, stay close to this thread as I suspect some other industry experts will reply who will help to confirm some products to use in the skimmer area - depending upon if the repair is required around the outside of the skimmer mouth or inside. If I missed any other area of considerations, they'll point them out.
Last thing. If that pool has been stagnant for a week with no movement, you might consider adding a little regular bleach and manually brushing it around just to help keep it from becoming a swamp. I know our water temps are still cool around here (mine is still under 60), but you don't want to see it getting crazy green on you.
Good luck!
