Sorry for your loss.
As you found out, locking it only deters the "honest" thieves, the one who look for easy scores.
Parking at a business also does not do anything for security unless it is locked behind barbed wire fences AND has 24/7 guards on duty.
Businesses tend to be very good targets due to the fact that once closed for the day the thieves will have 12hrs or more to do what they want because the property is unattended.
A very good chance that even if you took one or two tires off you may have come back to find the scorched remnants of your trailer done out of spite.
Determined thieves may also take that as a "challenge" and find a way to haul it away. Basically they could take one tire from the other side and put it on the matching axle.
I wish I could say something to help you at this point but really you should find a much more secure place to store your next trailer...