I am part of a group of scientists working for a private military research company. We were presented with a substance that we were able to make into a powerful weapon, but we failed to replicate the substance itself and we used up all of it. The weapon we made included an artificial intelligence, about as smart as a child, and we grew very attached to it.
The thing is, our nation is in the middle of a war and the enemy has made a sudden push forward that encircled an area - including the city we're in. We have debated what to do with the weapon since we've been cut off from command and have no orders for this situation. We don't want the weapon to fall into enemy hands or destroy it since we care about it. The war has been going for some time, so we know how the enemy operates. They will allow civilians to cross the border, but they are searched and all valuables are confiscated. The weapon at its most compact can fit into a cubic crate 1.5 meter (5 feet) at a side, so there is no chance of getting it out undetected. The AI can enter a sleep mode, so it won't get lonely.
We have decided to hide the weapon somewhere in the city or close by and go get it later. The tides of war are slowly turning, and we expect our side to retake the city eventually - but that might take years, or even decades. The enemy will be poking around in the city while they can, but they have no reason to know of the weapons existence or go look for it especially.
Where do we hide it so that it will not be found for decades of enemy occupation, but will be recoverable and intact when our side controls the city again? The city would be modern-day Prague, or another city of similar size and advancement. We have a week to prepare before the enemy takes the city, a few days less if we want to hide it somewhere in the country.
Hard mode: the weapon needs electricity to not die, about as much as a desktop computer, but has batteries that take a day to charge and last a week.