The containers vary by type, but stay within a specific range for that specific location. The relative level of the dungeon doesn't matter - certain items spawn of certain fixed ranges, based on what the designers put in at the time.
Typically, you're looking for the wood & metal random chests to pick.
STATIC containers seem to change randomly with each relock. I had the static chest in the Serp's hold warrior's guild go level 4 on me, and I was able to sit there, breaking picks on it, on one character until I hit the minimum to pick a level 4 chest.
Random Barrels, kegs and crates are level 1-2.
randlom Chests are level 2-4.
For example, the Destard crate area typically has 3 random chests in it, IIRC - 2 of one level, and 1 of another. Similarly, Gypsy and brigand random spawns will have 1 each of two different levels.
You can also find someone with a bunch of paragon chests that need opening, and pick them.
If you can gain access to the event area that has been used several times in the past (I think during the Lenshire events and others - an underground area with LOTS of static chests and an island), it can be an excellent place to train. Most shards had it password accessible from a point in the Delucia passage, IIRC, and left it open after the event. In fact, several EMs had events messed up from the (different) password from the PREVIOUS use of the cavern still working, and people jumping ahead of them, so each cavern should have 1-2 working passwords.
