Bath Tub Leak

We have known that the boys upstairs shower had been leaking for a while.  We saw a spot on the ceiling and had the boys stop showering and then had someone come out and regrout the tile and caulk around the tub and floor.  

This has not gotten any better, the spot got bigger so Justin decided to knock a hole in the ceiling to see if we could find out where it was coming from.  We had the boys shower and water started dripping, not from the pipe but under the 2x4 on the right side of the pic below.   We cannot figure out what is going on.  We had the boys start showering in there again and did for a week with no problems, 4 showers a day and then a week later Justin walked inside and water was pouring out when Jonah was showering.  We had him stop and now we cannot get the problem to recreate so we cannot call a plumber out until we do.