When you drain your vacuum tank you will also find a small amount of water the 1st time and this is normal.On an alcohol engine you will find even more water.

PS there is no need to worry about forgetting to drain your tank.If you do it will remind you when it needs draining!
I have the Wife save the small butter bowls to drain mine in.I like to leave my pit space as clean as it is when I get there.At the end of the day you can put the lid back on it and duct tape it and put it in the trash can.Some may say not to put it in there but I say thats better than draining it on the ground.