forgot to return equipment

so i got laid off back in august and the company sent a prepaid fedex box for me to send back my equipment. i sent back my work laptop and all the chargers and wires and my headset and i THOUGHT i also returned my work phone... but i just found it in a box in my closet where i left my wfh set up equipment...

i already received my severance in a lump payment back in september. i just reread my separation agreement and it said that they wouldn't send me my severance until they received all the equipment back and i would assume that they already had a list of what equipment i had since IT had to issue it to me.

the agreement also said that in the event of a breach or threatened breach to any of the sections of the document including the equipment return section, the company is entitled to obtain equitable relief in the form of a temporary restraining order, temporary or permanent injunction, or in the form of specific performance. the company can cease separation benefits or you agree to immediately repay to the extent permissible by law.

so... what do i do? do i just email HR and let them know of this VERY HONEST mistake? i literally had zero idea i still had it like i SWEAR i remember tossing it in the box. i just think it's surprising that they haven't reached out about it yet bc it's not like they give work phones under the table like there's so much documentation