"Under" doesn't really modify anything there. It's a preposition and not an adverb. It tells you about the spacial relationship that exists between "his money box" and "his bed". Namely, it tells you that his money box is located or can be found under his bed.
Usually, the usage of "< some amount of time > ago" is as an adverbial phrase, used to modify a verb phrase.
In the first sentence, "ten hours ago" refers to how long ago the police found the footage.
The second sentence is slightly trickier, because a strict grammatical interpretation of the sentence is that it asks the listener whether, a hundred ...