Recommended Security Measures for Unlocked LTL Truck Deliveries

We receive LTL trucks that are not locked. Wah tare acceptable devices (locks for LTL) or systems that they could use.

no reason the driver can put a padlock on the doors

Check into the LTL and see what their policy states regarding transport of food and food security.  A lot of them likely will refer to the fact they make many stops, so drivers are constantly checking and aware if someone were to tamper with the trailer in transit.  That then doesn't account for the risk of 10 different operators accessing the trailer during its last leg to your facility, doesn't address open cross docking at the LTL carrier facilities, so on, so forth.

When we ship LTL, we use security tape on the packages and essentially have to rely on that as our measure, and we expect it from the suppliers who send us LTL's.  LTL is less than ideal, but it's a reality of business and the only thing I've ever been able to think to defend product during is to make sure there are security measures on the individual pallets and cases.