What are indications that a tank is getting low, other than the gauge, when you are underwater or do you take that last breath and that's it?
The primary indication is generally going to be an exponentially increasing work of breathing (WOB), possibly for 1 or (maybe even 2 breaths), then you find yourself sucking on a vacuum. And the relative abruptness with which you reach the 'vacuum' stage increases with depth.
have run out of air after surfacing from a dive while waiting on a boat pickup (drift dive.) As I recall, although I knew the gauge showed empty, I was able to suck in small amounts of air and the air seemed very dry (or maybe my mouth was just becoming that way.)
I am not aware of a particular reason why the air should seem to be very dry at the 'end' of a cylinder. Maybe, others have an explanation. Underwater, you should feel breathing becoming more difficult and then find you are sucking on a vacuum. Depending on where you are in remaining gas supply, you may feel the difficulty increasing at the end of inhaling one breath, then realize you are OOA as you try to take the next one. Or, it can all happen during one breath (particularly at greater depths, as noted). The abruptness may be subtly influenced by the type of regulator (balanced, unbalanced), the degree of diver's exertion, etc.
I have experimented with breathing (pony) cylinders dry at depth (100') and found that the increase in WOB is barely noticeable before I reach the 'sucking on a vacuum' stage, using a regulator with balanced first and second stages. YMMV.
In OW training at least one agency has students breath normally underwater while the instructor turns off their gas supply (for essentially an instant) so that the student can feel what it is like to run OOA. But, that is done in the pool, usually in the 5-10' range, and the effect is a bit more gradual than it would be at 100', where even if the diver's tidal volume (volume of air inhaled - and exhaled - during normal respiration) remains the same (as under resting conditions at the surface), the absolute volume of compressed air used with each breath is increased because of the effects of pressure. So, if the resting tidal volume for an adult is considered to be ~500mL (and is greater during exercise, and 500mL is therefore used only for simplicity of illustration, not as a realistic value for divers), then each breath at the surface reflects use of ~0.018 CF. In contrast, each breath at 100' would reflect consumption of 0.07 CF. For an AL80, that would mean at the surface you use 0.7 psi with each breath, while at 100' you use ~2.7 psi. And, that is using a resting tidal volume, which is conservative, even for relaxed diving. Tidal volume does increase with exercise, and divers are taught, as well, to breath slowly AND deeply (i.e. greater tidal volume), so the values above may be almost absurdly low. But, these numbers also illustrate why you experienced a situation where the SPG was showing an empty cylinder (zero air remaining) and you were still able to get a small amount of air - the SPG is simply not sensitive enough to reflect single digit psi differences.
Always thought that if I experienced that underwater, I would try and get buddy's/DM's attention and signal "out of air" and hope they were ready to surface!
If you will pardon a little bit of sardonic humor, 'HOPE is not a strategy.'
But, if you were to experience an OOA situation U/W, you should use the standard procedure as you noted - get the attention of your buddy / another diver, share their air, and - whether they were ready or not - execute a normal ascent. If they were not close by and you were relatively shallow, execute a CESA, deeper it would be a buoyant ascent. In reality, as I am sure you already know - and several posters have already mentioned - no diver should ever go OOA, if they are monitoring their air supply appropriately.
