In a plastic bag inside a lead box at the bottom of and painted the same color as the inside of the toilet water tank. What, doesn’t everybody do that to keep their car or other locked valuables safe? It’s not like your college roommates can “borrow” those things without the keys right? As an extra bonus, you’ll never forget where you’ve put those keys. And don’t ask me where I keep the spares!
The first eye-level flat surface in your home where you did any sort of activity upon last returning home. So a sight line a wee bit higher than you’d usually look for them. Or, in a pocket inside your bag you usually keep zipped that you have basically forgotten about that was left open just enough to let the keys fall into.