There is really no reason why your PC would suddenly overheat. If it's going to do it when you're away then it would have done it while you're there, and there's nothing you can do about it either way.
It won't hurt to increase your fan speeds, so if you want piece of mind then do that. If they're plugged directly into the PSU then they should be on full anyway. Sometimes SpeedFan just doesn't report certain system fans. It depends how good the support is for your motherboard chipset.