Hiding is making my blood pressure explode

Grant is now 4- but I am having some doubts about him reaching age 5. He has recently started hiding in the house. He has progressed from hiding in predictable places, to more unique, steathy locations. AND he will not come out when you call his name.
This has what has been going on the last week. During the following occasions, we were not playing hide and seek- in fact these were at very inopportune times to hide... I hope my neighbors can't hear the yelling through the closed doors.

This started innocently enough the other night while getting ready for bed- he hid behind a toy chest in the playroom. I called and called, but not word from him, and no coming out. He's actually able to stay quiet. I think it took about 5 minute to find him, but I was pretty sure he was still inside.

Later that evening, after he had gone to bed, we couldn't find him in his room- or anywhere obvious. I went in to Grace's room and "she hadn't seen him", we called, we tried to remember if we heard the door alarm opening, looked in the garage, outside etc. The more this goes on the angrier I am getting- finally, we go back into Grace's room and he is hiding in her be under covers- and she thought it was OK not to tell us when we were looking. I think she now understands how incredibly frustrating this is.

The hiding occured again last night before bath, it took about 10 minutes to find him, this time in a different place.

BUT, this morning is what made me absolutely crazy. It is morning, and its a school day which means time is precious. After he eats breakfast, he is supposed to be upstairs getting dressed. I go up after a bit to check on him- and of course he is not anywhere to be found. I call and call, and get no response. I start raising the threat level in the house, and again no response. I am so mad at this point I don't think I can speak. Finally, he comes out- he has been hiding at the bottom of my bed, and we've now wasted atleast 15-20 minutes of precious time in the morning. I think that child might have vinegar flowing through his veins before he reaches adulthood. He had to come down and drink some vinegar again- for hiding and not comign out- and I kept adding a 1/2 tsp to the cup after each 5 second period... I ended up adding 1 1/2 tsp by the time he got down there. Amazingly they both seem to know that we're going to increase the amout of vinegar etc with the longer they take to drink it. I have no idea how to put the fear of you know what in him, because he just turns and laughs at you. He thinks everything is funny.

I explained that it is only OK to hide if he is playing hide and seek with OTHER KIDS ( not just his mom or dad), and its a game that everyone agrees on.

If this hiding thing doesn't stop immediately, I think I might have a cardiac arrest from high blood pressure. AAAAAAAAAHHHHH!!!!!!!!!!!!