It's not theft, it's evidence.
As it isn't illegal to own a RADAR detector, just to use or transport one (except under specific circumstances), you're entitled to its return upon a finding in your favour, in court. For reference, this is the section of the HTA involved:
79. (1) In this section, “speed measuring warning device” means any device or equipment designed or intended for use in a motor vehicle to warn the driver of the presence of speed measuring equipment in the vicinity and includes any device or equipment designed or intended for use in a motor vehicle to interfere with the effective operation of speed measuring equipment. 1996, c. 33, s. 12.
Speed measuring warning device prohibited
(2) No person shall drive on a highway a motor vehicle that is equipped with or that carries or contains a speed measuring warning device. 1996, c. 33, s. 12.
Powers of police officer
(3) A police officer may at any time, without a warrant, stop, enter and search a motor vehicle that he or she has reasonable grounds to believe is equipped with or carries or contains a speed measuring warning device contrary to subsection (2) and may seize and take away any speed measuring warning device found in or upon the motor vehicle. 1996, c. 33, s. 12.
Forfeiture of device
(4) Where a person is convicted of an offence under this section, any device seized under subsection (3) by means of which the offence was committed is forfeited to the Crown. R.S.O. 1990, c. H.8, s. 79 (4).
Penalty
(5) Every person who contravenes subsection (2) is guilty of an offence and on conviction is liable to a fine of not less than $100 and not more than $1,000. R.S.O. 1990, c. H.8, s. 79 (5).
Exception
(6) Subsection (2) does not apply to a person who is transporting speed measuring warning devices in sealed packages in a motor vehicle from a manufacturer to a consignee. 1996, c. 33, s. 12.
Sale of speed measuring warning devices prohibited
(7) No person shall sell, offer or advertise for sale a speed measuring warning device by retail. 1996, c. 33, s. 12.
Penalty
(8 ) Every person who contravenes subsection (7) is guilty of an offence and on conviction is liable,
(a) for a first offence, to a fine of not more than $1,000; and
(b) for each subsequent offence, to a fine of not more than $5,000. R.S.O. 1990, c. H.8, s. 79 (8 ).
In the OP's place I would write a politely worded letter, to The Crown, stating something like "Pursuant to Section 79, part (4) of the Ontario Highway Traffic Act, I am requesting the return of, or compensation in the amount of $XXX.XX for, the speed measurement device that was seized on (Date), under Offence Notice (Number), as disposed of at ( State Court Location) on (Date)."
... or words to that effect.