Q: Can police legally unlock and search my glovebox without a warrant

A: You're best legal challenge would be the reason for the initial detention. In Texas a police officer has to have reasonable suspicion that a traffic violation has occurred to pull over a vehicle. If the only reason the officer had to pull you over was the expired registration sticker it may well be that the stop was Illegal and all of the evidence found form the stop is "fruit of the poisonous tree" and should be suppressed leading to a dismissal. However it isn't agreed upon throughout the state that the registration sticker is an invalid reason for a stop. Governor Greg Abbott has announced that the temporary waiver of certain vehicle title and registration requirements will end on April 14, 2021. So until that time you cannot be convicted of the offense of displaying an expired registration sticker, but it is an open question undecided by the courts as to if an officer may begin an investigative detention. In Harris County the District Attorney's Office has issued memoranda to the assistants to this effect but in other counties you may be dealing with a different situation.

As far as the searching of the locked glove box, California v. Acevedo from the United States Supreme Court holds that an officer may search the entire car inside and out including locked containers like a locked glove box if the officer has probable cause that evidence of a crime will be found in the vehicle. In this instance you seemed to indicate that marihuana was taken from your person prior to the search of the vehicle. Depending on how any why your person was searched this could lead to enough probable cause to search the locked glove box based on what is called the "automobile exception." The automobile exception would not save the legality of the search if it is ruled that the stop was invalid because of the registration issue.

A: Expired registration remains an offense in Texas which gives an officer the right to stop a vehicle. The odor of marijuana in the vehicle gives the officer the right to search the vehicle. Was the vehicle registered to you? There may be an affirmative link issue (it's a legal thing; get with an attorney). Good luck.