There's really no point in telling us you can't find the leak. Inflate as much as possible and push sections into a tub of water. It helps if you do it in a quiet area as well. If you still can't find the leak then you will need to carefully inspect every inch of the inside of your tire and rim and hope to find the cause. Also, I would think a slime tube that leaks would leave some residue at the point of the leak. I actually would recommend putting in a non-slime tube (and carrying a spare of course) to see if you get another flat and if it is then easier to spot the leak.

Also, using the term "popped" implies a quick release of air (blowout). It's more common and clearer to say the tube leaked or went flat when it is a slow loss of air.