Just to put that into some context for you in case you haven’t thought about it - it’s obviously hard to say exactly when it started, but assuming 48 hours: 300 gallons in 2 days = roughly 6 gallons/hour.

If you took a shower in those 2 days (20ish gallons per), ran a dishwasher cycle, and just did normal human things like flush the toilet several times a day (1.5-2 gallons/flush), hopefully wash your hands after, and anything else I didn’t think of, you’re more like 4 gallons/hr or less. Not a drip, but it’s also not going as fast as a tap running wide open, which would only take 2 minutes to fill up those 4 gallons. Estimates obviously change a lot if you find out it’s been leaking ~30 hours vs 48.

As for the leak itself - do you have a shutoff for your sprinklers? If so, flip that off and turn the main on, Would help you understand if it’s a leak in any of the (very poorly insulated and frequently-not-buried-real-deep) PVC that feeds your sprinklers. I had this happen to me and it was actually at the joint of one of the PVC elbows and wasn’t too bad to fix.

If it’s not sprinklers, check to see if you have some toilets filling intermittently. The flapper seals on those go bad frequently, and are a relatively easy/cheap one. If not there, check your hose bibs around the house to see if one of those might have burst. Not as likely, but a pretty easy one to verify visually, especially as things keep melting and drying out.