Exactly.

I would also bury it under 2' of dirt if possible as most workers and kids won't even get down that far unless you accidentally bury it next to an existing pipe!

I can think of tons of places in the city to bury such a pipe w/no one looking. Even if they are looking 99% chance they are not going to come over and ask and less than .0001 % that they will actually dig it to investigate further.

Be careful if you dig a hole in the city, in most towns it is illegal unless you call the local marking service first.

I will check with a couple LEOs, but I think if you were to bury something like a pipe or bucket anywhere in a city:

1. Local druggies or homeless would dig it up and make off with it in a heartbeat.

2. LEO or union workers would stop you unless you did a stealth, dead-of-night job.

3, Digging any such thing up would subject you to the attention of the above groups.

There are eyes everywhere and the first thought about any out-of-the-ordinary activity: terrorist.

How much does the smallest size of "U-Store-It" type storage run? Many of them have only one gate between you and your unit. Pick one that you could still open if the power was out.

Can you imagine the reaction if a city crew found a buried 5G bucket, lid carefully sealed? I don't think I'd put any ID in there. And probably try not to put any DNA, either . . . .

The next question is about what you intend to bury?
If it is big enough for you to use a backhoe it gets even easier.
You will need signs and barricades then, but your visibility is so high that you become invisible.
Just be careful to not be near terror targets.

Two of those folding "Men Working" signs, a sawhorse style barricade, coveralls and a pick also let you open manhole covers. If you choose properly you can find one that only gets opened once every 50 years. Making your deposit is easy, just put your stash in place and replace the cover.
Recovery is easy, you just lever the cover out of the way with your pick or bar, grab your stash and leave.
Remember that if you need to enter a confined space you should ventilate it and gas test it first.

Edit: If a city crew found your bucket they would either report it as found property or just divide it up among themselves.
So long as there are no guns in it there is nothing they would care about.

This. Very much so. Also, as Scafool noted, no guns. No weapons. Also no money. Absolutely nothing newsworthy. On discovery, a deliberately hidden cache of any sort is going to attract all kinds of attention from the 'War on Drugs' to the 'War on Terror.' Not quite the attention you want when you just want to keep a silcock key around.

Also, stashing various items (whatever they are) in 'public areas' (including National Forests if left in place over 14 days) is unlawful. Don't do it.

Well, this is a fun question. People who are into geocaching get involved in this all the time--making and camoflaging caches, hiding the caches, finding the caches. I dunno, how many geocaches have you folks accidentally run across? Before my wife and I started looking, I never ran across one, even though there are hundreds of thousands of 'em hidden all over the world, some as big as 50 cal. ammo boxes. And almost none of them are buried. In fact, on my Garmin Colorado 300 GPS right now, I have 500 geocaches entered, and that's within 15 minutes by car of my house in SoCal. So, if you wanna hide something, and find it later, you might want to ask a geocacher.

Ok, how do geocacher's hide something so you can find it later?

geocaching.com

Geocaching rules on the site mentioned require obtaining permission and specifically exclude burying.