So... this was in my oil drain pan...

[Dynomite]

I say remove the oil pan and check the windage tray to see if there is any place a 1/4 NPT plug can fall through.

If no.......then that plug has been in oil pan for a long time or in your oil drain pan for a long time and nothing to do with your perceived small drop in oil pressure (which you just noticed recently).

If that 1/4 NPT plug can drop through the windage tray........then check that bottom crankshaft plugs are intact. If they are intact......The Cams each have a rear plug about 3/8" OD, 1/8 NPT (.405" O.D.) unlike your 1/4 NPT Plug (.540" O.D).

And.......when you say idle pressure is 25psi-30psi (Hot)....what is your idle rpm?
Makes a difference in oil volume output. And.....what is your oil weight?? (Maybe you are using light weight oil for winter).