Inside the Anti-Magic cell of the Old Tower.
Marko finds a chisel. Upon examination of the trunk (which is all made of steel -- no wooden parts) the weakest link appears to be the steel lock plate as Marko surmised. Althea figures she should be able to smash/bend the one inch think steel enough to get the padlock off, still locked but with the expensive lock mechanism intact.
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padlock's lock = DC40 vs OpenLock ("Amazing" level lock)
padlock's break/smash = DC28 vs Strength check
padlock's HP = 30 hp with Hardness 15 (standard for Mithral)
Steel trunk's lock plate "eyes":
break/smash = DC26 vs Str check
HP = 30, Hardness = 10 (standard for steel).
The "eyes" are hard to hit with a weapon because of the cunning way they are covered. You could use something like a chisel and hammer but that would do only 1d6 + str bonus. You can hit the padlock itself with any weapon. You can use a lever like a crowbar to help strength break the padlock (+2 circumstance bonus) but not the "eyes".
Finally you can try to smash open the trunk. It's hardness is 10 (steel) and it has a mere 10 hp so that's certainly the easiest way in but would probably break a bunch of stuff inside.
Remember that you can't use magic to help because you need to have the trunk in the anti-magic room.
The lid of the trunk is flat and there are no hinges visible. There are two handles on the ends and one on the lid.