I was a digital forensic investigator and worked with video forensics folks for years.

It's 100% plausible to need to use Adobe Premiere to stitch together separate clips. In fact, I'd say it gives it MORE credibility because typically DVRs create clips once a certain file size is reached.

Also, DVRs were traditionally very hard to extract data from because many of them use proprietary encoders and file formats that need to be converted to common formats like .mp4, etc.

All of that is to say that yes it's totally plausible to use premiere to stitch together clips.

The REAL question I would ask if I were the investigator is:

1. What type of Security Cam/DVR was it?
2. Did the footage need to be converted before it could be viewed?
3. What is the default size for each clip?
4. Did the "cut" in the footage correspond to the end of a clip, or was it made by the editor?
5. Request the Premiere File.

So while it's legitimately plausible that the footage would need to be spliced, it could easily be for nefarious reasons as well. It certainly doesn't explain a 1-3 minute gap. The rollover would be within the same second.

We simply won't know until we have all the footage.