If you don't turn off a Surface Pro (eg just slap the keboatd shut) and put it in, say, a rucksack, will it be likely to heat up (since there won't be anywhere to vent any heat)? Would it even still produce heat in standby/sleep?
If you just slap the keyboard shut, the Surface Pro will be in hibernation mode and it won't be turned on. It won't consume any power (except for the RAM), so no, it won't heat up and the vents won't be turned on. It's the same with any notebook that you put in standby/hibernation (by just closing it).