M.W.
2 1/2 is NOT too young to expect him to answer you and not hide. We farm and ever since our kids could walk and talk (about 18 months) they KNOW that when I say, "Where are you" to ANSWER! They usually come running and smiling and say "HERE I AM MOMMY!" They know it is important to be safe ESPECIALLY when we farm and they are in the barn with us.
Sit him down and tell him the rules before you leave. Tell him what you expect. Have him REPEAT it back to you. Tell him what will happen if he disobeys. Have him say YES MOM so you know he heard you. And most important, FOLLOW THROUGH!
We count sometimes with our kids when they are being testy becasue they think it's funny. We NEVER get to 3! They know there would be consequences. They would either get a spanking, time out, or SOMETHING.
This is dangerous behavior. If he won't stop after laying down the law, I would stop going to parks for awhile OR leave him behind with a sitter (doing something not fun like nap or quiet time) while you and the 1 year old go have fun at the park. A time or two of that and I think he'll shape up!