Fitted an STI Intercooler to my MY01 WRX recently, everything fine until today when it starts making the aftermarket dump valve whoosing noises!!

Had a look under the bonnet, and there was an allen bolt in the side of the BOV, which is now missing! It's either worked it's way loose and fell out, or into the pipe.

Firstly, what's this hole for? and what should I do next?

 

Removed the BOV and replaced it with the standard WRX one, which seems to be identical, apart from this hole!

After closer inspection, this could have been a DIY job by the previous owner, possibly to fit some sort of gauge?

Could someone with a newage STI or STI intercooler have a look.

As you look from the front of the car, it's on the left hand side of the BOV, just above the hose and clip, going under the intercooler.

Would like to know where the allen bolt went though, hope it worked it's way out and not into the pipe, which looks like it goes down to the turbo.