I then also realized that there were 7 people now in my squad instead of the usual 6. I asked my squad members if they saw they 7 members and they replied that they didn't, neither were they able to see him on the map.
I could see this 7th player (so happening to be camo_jnr_jnr) on my map. I also noticed he had an AT weapon, then later switched to AR (which already existed in my squad.) I could identify this player on the list of players for my team. My squad members reported him as being on the enemy's side. See below.
To see if switching out of the squad would solve the issue, I created a new squad. The enemy player remained in the squad alone and still visible to me on the map. My SMs reported that the old squad had been completely dissolved. See below.
I asked one of my SMs to try and create a new squad to replace the one we dissolved. Someone managed to remake the squad with no difficulty. To me they were in the original (bugged) squad I had dissolved earlier (no screenie), but they reported being in a new squad.
I solved the issue by merely disconnecting from the server and rejoining. No need to restart the entire game.
As a PS: Our squad only observed the obviously-a-cache location (AT, AR = RPG, PKM) and eventually some random squad came and took it down without our coordination.
B4TM4N