jetphotos.net/viewphoto.php?id=6147898&nseq=33
seems stange
not particularly - as mentioned in other threads these and many other civil airliners are mostly involved in troop flights and we are at present working many over the UK usually under RCH (Reach) callsigns.
Some go Stateside direct to places in eastern europe/the gulf region and then beyond (!), the 738 likely States-Shannon-eastern Europe (with an upgrade unlikely).
IN (UK)
N321TZ was also in Munich the previous day.
Here’s some European 737-800 flights for ATA. All are military charters.
AMT8049
AMT8043 (from Gander but probably originated in Europe)
AMT8002
AMT8018 (from Gander but probably originated in Europe)
There are at least 35 airline companies who have contracts with DOD for flights when military aircraft are not available known as the Civil Reserve Air fleet,they move personal and cargo on an as needed basis.
Civil Reserve Air Fleet Fact sheet
This site shows the aircraft allocated to the CRAF program.