A Mechanic was informed a forward nacelle pylon panel on #1 engine departed a DHC-8-100 aircraft on take-Off roll after he had completed a preflight Service Check. Work interruptions involving other maintenance tasks contributed to the upper 'horse collar' panel not being latched prior to take-off.

Narrative

I completed the preflight Service Check while Mechanic Y did an unscheduled [fuel] nozzle change on #1 engine. I also changed the power take-off unit (PTU) check valve for a function check. We jacked the DHC-8-100 [to do an] Ops Check of the PTU check valve. Mechanic Y went back to working on the [fuel] nozzles. I completed the Operational checking of the PTU check valve and closed the panel. While I was on top of the wings I completed the panel inspection for the preflight Maintenance Check.Once I got on the ground; I removed the hydraulic and electrical AC carts and finished up on the panel inspection from the ground. I stamped-off on the Preflight Check Work Card knowing that Mechanic Y; doing nozzles; would have to complete his work card for closing the cowling. The aircraft was still on jacks when Mechanic Y finished the nozzles and I finished looking over the paperwork while waiting for help to get the aircraft down off the jacks. Mechanic Y pulled the engine stand out of the way and I assumed he closed and secured the cowlings and he helped take the aircraft off jacks. The horse collar panel wasn't known missing until the aircraft landed in ZZZ1. Mechanic Y doing nozzles was being interrupted to help with other tasks; like jacking the aircraft. There is going to be a meeting with all the mechanics looking for suggestions.