A DHC-Q400 Captain reports about an air turnback and return to field after Takeoff due to the Nose Gear would not retract. The Nose Gear Ground Lock was still engaged and the external Lock handle was found popped out. Aircraft does not have any warning indication when handle has popped out.

Narrative

After we had already taxied out on a Maintenance Reposition flight and had to return to the Hangar due to a 'roll spoiler outboard grnd' light; we waited for Maintenance to troubleshoot that problem. We left the aircraft to use the bathroom and get the new Dispatch Release and the aircraft was moved by the Maintenance personnel and run up to continue the troubleshooting process. The Nose Gear Ground Lock had apparently been engaged without our knowledge and not disengaged after the tug was removed. After the initial problem was fixed and signed off (turned out to be a loose cannon plug inside a panel on top of the left wing) the First Officer (FO) and myself looked around the work area and made sure there were no chocks and the pins were stowed in their proper location. Neither of us caught the Nose Gear Ground Lock handle. We started engines and turned out for ZZZ1 again. We taxied to RWY 30 at ZZZ and took off. The Nose Gear did not retract. I decided to return to ZZZ after having the FO run the closest Checklist which said to extend all gear and if it goes down leave it down. All three gear were down and locked and I kept the airspeed around 180 on the short trip back to ZZZ. We did not declare an emergency as we had all 3 gear down and 3 green lights. We landed uneventfully and taxied back to the Hangar. The Maintenance personnel immediately noticed the Nose Gear Ground Lock handle out and pushed it in. We shut down and talked to our dispatcher and wrote up the issue which was signed off. We got more fuel and a new Dispatch Release and departed.... uneventfully.... to ZZZ1 finally. The Nose Gear failed to retract after Takeoff. The Nose Gear Ground Lock was engaged after the FO did his walkaround. We both missed this item on our way to the plane and during our abbreviated walkaround to assure the chocks were pulled and pins were stowed. The Nose Gear Ground Lock is something we never deal with in normal operations because the Ground crew is involved in the pushback and they are responsible for that item as we are in the plane with the doors closed.... I had picked up two other planes at the ZZZ hangar and never had this issue before. We ran the Checklist and notified Departure that we needed to go back to ZZZ. Also sent an ACARS to Dispatch to notify them and Maintenance. Pay closer attention during irregular Operations as to position of Nose Gear Ground Lock handle. I was very focused on chocks and pins; but did not notice the handle. Communicate with Maintenance when picking up an aircraft at the Hangar to ensure the Nose Gear Ground Lock handle is disengaged after the tow bar is disconnected. Also have Maintenance obtain and use a headset so there can be voice communication during these irregular Operations. Don't rush if you get behind....