These guys are probably SAS and SBS. The special ops forces are fanatical about keeping their identities secret. They could even be carrying special eqipment that they don't want anyone else to see.
You can guarantee that the media would have their photos plastered all over the pages of the newspapers if they are rescued by Mexican authorities.
This means a lay-off from active service for at least a couple of months.
SAS often go into countries on tourist Visas. There's a fine line between what they do for work and what they do for enjoyment.
The Ministry of Defence said the men were there during a combined services adventurous training expedition and were not on exercise.