A spellcaster uses Disguise Self to pass as a simple bodyguard. In particular, the quarterstaff that she uses as a spell focus, which normally looks like an obviously magical item (runes, faint lights, the works), is disguised to look like a simple martial weapon (the kind of quarterstaff that a monk would use).
She is asked to drop her weapons in a box before meeting a mob boss. She doesn't want to reveal that she is a spellcaster.
At the moment where you cast the spell, Disguise Self applies to belongings on your person. What about afterwards? Would the quarterstaff, once put in the box, immediately take back its previous appearance?
If the answer is "yes", could Silent Image be used to maintain the illusion? I would want to create the exact image of the fake quarterstaff "around" the actual quarterstaff to keep it camouflaged. Is that a possible use of the spell? Would someone picking up the quarterstaff, without examining it too closely, immediately notice the illusion, assuming the weight/material of the illusion is similar to the real one?
Is it still possible to control the illusion (in particular, to keep it around the quarterstaff if it moves) even after the spellcaster has dropped her spell focus?