The question you pose relates to the theft component of robbery. If the taker believed the item was theirs, then the mens rea for theft was not present, and therefore, there was also no robbery.
See Criminal Code, s. 343:
Every one commits robbery who
(a) steals, and for the purpose of extorting whatever is stolen or to prevent or overcome resistance to the stealing, uses violence or threats of violence to a person or property;
(b) steals from any person and, at the time he steals or immediately before or immediately thereafter, wounds, beats, strikes or uses any personal violence to that person;
(c) assaults any person with intent to steal from him; or
(d) steals from any person while armed with an offensive weapon or imitation thereof.
To "steal" is defined to mean "to commit theft."
Theft is defined at s. 322:
(1) Every one commits theft who fraudulently and without colour of right takes, or fraudulently and without colour of right converts to his use or to the use of another person, anything, whether animate or inanimate, with intent ... (a) to deprive, temporarily or absolutely, the owner of it, or a person who has a special property or interest in it, of the thing or of his property or interest in it ...
(4) For the purposes of this Act, the question whether anything that is converted is taken for the purpose of conversion, or whether it is, at the time it is converted, in the lawful possession of the person who converts it is not material.
The hypothetical you raise goes to the mens rea of the underlying theft, not anything special relating to the heightened offence of robbery.
The mens rea for theft is (R v Rowan, 2023 ABKB 164, para. 46; citing R. v. Lafrance,  2 S.C.R. 201):
- intentionally taking a thing; and
- knowing that this will deprive the owner of the property without any lawful justification.
If the taker believed the item was theirs, then the mens rea for theft is not present (R. v. Senior, 2021 ONSC 2729, para. 269):
The taking or converting is not done fraudulently and without colour of right if the person taking it did not know that he was not legally entitled to take or convert the property. Even if mistaken as to his entitlement, the person must be found not guilty.
Depending on the manner in which the item is taken, the act could still constitute another offence (e.g. plain assault) even if not theft or robbery. And as always, the actual owner would still have a civil remedy to have the item or its value returned.