I don't think they dig with pick and shovel anymore, the shovel is usually to lean on while they watch the backhoe

Anything you put anywhere in civilization you run the risk of it being found during maintenance. Anything you rent you run the risk of it not being accessible in times of need due to possibilities such as power out, the building/business closed, etc.

Some possibilities I have seen are networking with others then I store a box for you and you a box for me. Another is getting permission to geocache on public land and place 6 caches to be found and one to not.

LOL!!

Yes, always permission.

Around here, our storm drains are maintained by raccoons. You drive by at night and there are half a dozen sets of eyeballs staring out at you. Kind of creepy the way they reflect the car headlights. Anything you put down there better be encased in inch thick steel. Those raccoons can get into anything. On the other hand, I guess you could consider the raccoons to be your urban food cache! You don't even have to stash them in the sewers ahead of time. They just appear there on their own. You could fashion a spear to get them.

Hmmm, another idea would the migrating geese in the parks in the area. There are enough of those messy things to feed the whole town. You can walk up to about ten feet from them. So a bola made out of paracord and rocks would be all you'd need. Never thought of the raccoons and geese as being an "urban cache", but I guess they are.

Hey, can you eat coyotes too???!!! We've got those all over, but you'd need a rifle to get them. Not a technical problem, except for the pesky "no shooting" laws in town.

Purchase and hollow out three volumes of poetry by Ralph Greenleaf Whittier. Stash desired supplies within.Take to the public library. Shelve the cache books in the poetry section, or really, any section except new fiction or juveniles. Cache will remain untouched for centuries.

For greater volume, do the same with a complete set of "The Great Books" or a 30 year old set of encyclopedias.

Unfortunately, while I have plenty of storage space at work (several cabinet and option for a locker), some of the things I would like to cache are forbidden on the property. Ie, firearms, probably any ammo, pyrotechnics, pepper spray ... might be able to have a baton or something like that, and probably could get away with a backpack. But really, no point, since I have to drive to get to work, and my truck has a fair amount of stuff in it.

Never hurts to have a back up somewhere.

I thought you were talking about geocaching for a second...

In any case, make sure YOUR cache doesn't look like a geocache (which are sometimes in used ammo cans!) and seem to be hidden everywhere. You might inadvertantly put your safety cache in an area beside a geocache and be found accidentally. Just saying...

Secondly, locations such bridges, utilities, schools, etc. are also not good places lest someone find it and think it was a bomb and then call out the squad. That happens all the time in the geocaching world.

That's the most robust and the cheapest. But, short of complete PAW/TEOTWAWKI/WROL/The Big One/societal collapse...
www.storagewest.com
www.publicstorage.com
www.uhaul.com
www.selfstorage.org
&c.

Do I need to elaborate? Storage rates for a small (5x5x8) closet start at about $35/month, and you can store a whole lot more gear in one then a couple of 17 gallon food storage containers hanging from a sewer grate. It's even cheaper if you split the space and rent with a trusted friend. Just keep up on your payments if you want your cache to stay off of TruTV or A&E.