I've just taken over a 401(k) plan and I found that no schedule p has ever been filed for it... It was my understanding that all plans need to file a Schedule P signed by the trustee. Now I deal with mostly db plans, so maybe this is common for 401(k)'s. btw... the company is a normal s corp

there is no requirement to ever file a sched P - that is an optional form.
see instructions "Schedule P MAY be completed..."
that being said, why you wouldn't want to file one is beyond me.

It's my understanding that the Schedule P sets the start date for the statute of limitations.....so no Schedule P, no statute of limitations - you may want to be sure that the prior administrator dotted all I's and crossed all T's!