It sounds like an intermittently leaking master cylinder, it wont be an external leak but the fluid bypassing the piston.

>> I had a look at the master cylinder and it didn't appear to show any major leaks near the pedal.

Is it losing fluid? Check the resevoir level. If it's low, you probably have a leak somewhere. Follow the hydraulic circuit end-to-end and when you find the leak, you have found the problem. I'd be studying the connection to the slave carefully, as that's what's been touched most recently.

If you cannot see any evidence of leakage and the fluid level is low, it's possible that whoever replaced the slave neglected to top it up after bleeding(!) and you are getting air in the system. Worth topping up, bleeding the system and monitoring to see if it drops again.

If no leak and fluid level is ok, then there are two possibilities that spring to mind. Cheapest is that the return spring in the slave cylinder is dodgy, so it's not returning fully (seen this more than once on a variety of vehicles). Change the slave as a first attempt at a fix. If swapping the slave doesn't fix it, then an iffy internal seal or valve in the master has to be favourite.