Ball taken - ruling?

[JackInDaCrak]

Yesterday I'm playing on a muni course. On the 422 yd Par 4 14th, we wait to tee off until the group in front of us is clearly over 300 yards out.

I really get ahold of my driver, and it rolls up near the group in front of us, probably like 320+ yards total in center fairway (brag). Now, this is about a 1% drive for me... I don't hit 'em like that, ever.

The group in front is still hitting their approach shots, and they all do. I watch three of 'em walk by my ball, look at it, and move on. The fourth I see drive past it and pick it up.

I drive up to the green and ask these guys if they found my ball. They all say no, then two of 'em turn to a third and say, "maybe that ball was his dude" so he pulls my ball out of his pocket and hands it to me.

Obviously, huge KITN for this guy, but muni golf is, well, muni golf.
Do I just get my playing partner to agree on a spot for my drive and play from there? Or, should I ask this douchebag where he picked it up? I wasn't sure what to do wrt my next shot/drop/penalty/etc