Ive kept a few freshwater crabs before and they are excellent escape artists. For me, they would climb up my filter pipes, up ornaments..basically any way they could find to escape. I once even seen one climbing up the corner of my tank climbig up the smooth silicon on the corners of the tank and I stoped him halfway up. What I had to do was close every possible hole there was with small peices of plastic or carboard. I had a stock lid so there was open holes for heaters and such. Just to clairify, if the problem is them getting out and you finding them outside the tank, then you could try that. Or if the problem is that they are dissapearing and you think that they might be getting out but your not sure, then it could be possible that they are just hiding somewhere really well, my crabs used to hide for weeks at a time and I would think they had died or have gotten out but then about a day later I would see them crawling around.
Riley