It's a type U spell, so you can't cast it on an unwilling person or on anything they are wearing/carrying. That rules out their armor, unless they are not wearing it at the time you cast the spell.
The description says it goes on a location, which I would normally interpret as something immobile, but it also says "A marker can be placed on any inanimate object." That seems to indicate it could go on something mobile or movable, in which case I would think the marker would move with the object. Note the wording (and type) is unchanged from RMSS to RMU.
When in doubt, you can always require a spell mastery roll, perhaps at more of a penalty for something that is more mobile.