Usually, you diffuse situations like these by saying you don't want to sound like a jerk when in fact the other person is the jerk.
"Hey Boss.. I don't want to sound like an a-hole, but I lent you a phone a while back. Since I'm leaving the company, I was hoping to get that back."
For 99.9% of the normal, decent people out there, this will trigger a subconcious response in them of ...
"Holy crap, I totally forgot about that! You're totally not an a-hole for asking for your stuff back. In fact, I'M the a-hole for not giving it back to you sooner!"
For the other 0.01% of people out there, it will trigger a "Deny, Deflect, Defend" response in them, because either
1) they don't want to give the property back
2) they don't know where it is and don't want to go to the effort to find it
The "Deny, Deflect, Defend" strategy is what a-holes use to try to deflect bad behaviour they've done.
EG: kids griefing on minecraft
(admin) why did you grief this person's build?
DENY - I didn't do it!
(admin) I have logs showing you did.
DEFLECT - It was my kid bro / sis! They must have logged on as me.
(admin) That's no excuse.
DEFEND - Ok, look, I did it. But, it's because this guy stole some diamonds from me.
If you ask for your property back, and your boss immediately goes into DENY, DEFLECT, DEFEND mode.. it's best to just walk away. They're going to try to find some way to make life difficult for you, b/c they still have power over you.
EG's of responses to know your boss is going to be a butt about it:
"Are you sure you lent me a phone? I don't remember"
"Oh, well, you're going to have to file a report with HR to..."
"Oh, right.. your phone.. I'm gonna need time to try to find that.. uh..."
"I thought you gave me that phone. Why are you asking for it back?"
If you're on bad terms with your boss, they could use this as a way to make your life difficult before you leave the company. If you're on good terms with your boss, but they're just a d-bag, then they will simply see this as you making their life hard before you leave.
So, approach your boss, ask politely, and if they respond in such a way that you can immeidately tell it's going to be a nightmare for you to get your property back.. JUST WALK AWAY.
"Oh, hey, ok, that's ok, Boss. Just consider it a present then. No big deal. Have a good one."
Just walk away and take it as a lesson to not lend out personal items at work going forward. (Most folks learn this in school by lending out pens / pencils to folks and realizing a-holes don't return them.. so it's best to just never lend anything out.)