Teer Kerensky, on 31 October 2016 - 07:18 AM, said:
When the red triagle above enemy is empty, it means no one has targetted that mech. It can be within sensors of several mechs, as they just haven't targetted it. Same as non-targetted but within sensor range of someone else, will not display on your sensors if it's outside of your range.
...
Techically tagging doesn't require targetting, but usually to be useful you need to target it as well.(for example if you tag someone with ECM but he is outside of others sensor range, they won't be able to target him, only you will) And you can also target someone further away and tag someone closer who can and are targetted by others.
So, the enemy mech in quetion was probably outside of tag weapon range. Or sensor range.
Another option is, it was protected by two ECMs. Tagging mech will counter ECM, but only one. Tag counters all effects of ECM.
While most of your information is correct, there are a few points that are not complete here (unless I am also misinformed).
The hollow Triangle means it's a target you can see and possibly lock, but no one else on your team has locked. Not the other way around. If you wish to share data (such as the mech information and the lock itself), you need to have the target locked. This comes into importance in relation to spotting, such as to assist with LRM missile locks.
You need to do more than just see a target if you want your team to be able to also target them and know where they are. Your team will only be able to see and target the same target you have targeted (no matter how many you can see) if they can not see the enemy mechs. However, you can have a mech targeted and share that target data with friendly mechs well outside sensor ranges.
As for TAG in general use, it is the same as with locking targets and sharing target data. If one person can Lock and TAG (I will admit, I am uncertain if TAG will apply and let a unit be targetable by allies if you don't have that specific mech targeted yourself), than everyone on your team would also share that lock, be able to lock onto that target, gain the benefit of TAG (which helps lock missiles, such as LRMs and SSRMs) and overall the teammates range is not relevant to the ability to share that target data. (However, that range may make the effects of TAG useless, if it's outside their weapon ranges. LRMs only have a 1000m range, so TAG effects on a target outside that range for a friendly LRM mech would be unusable to them specifically, but they would know where the target is.)
In relation to TAG and ECM, TAG cuts through ECM in a passive manner. It doesn't disable ECM at all, but it permits targeting and missile locking. The key here is that the Tagger remains outside any enemy ECM affects themselves. If they find themselves near an enemy ECM and under their effects, than their TAG will not operate correctly.
Basically, if you have a lock on an enemy mech, your whole team can see where that enemy mech is no matter where they are on the battlefield in relation to you or them. Just looking at hollow triangles (just looking at enemies) is not enough for this to happen.
I've actually
written a rather extensive (though slightly old)
guide/explanation on spotting, LRMs and their interactions. It's long winded and a little disorganized, but it may be helpful if you(anyone) wish to read more into the subject.
PS: Sensor range is only relevant to locks you get yourself. It has no relation to locks shared by teammates. Just to try and farther clarify.