What bosses drop sigils

Yes. The Harvester and Stalker both have relatively similar spawns. Each one will give you a Death Mark and send a message accordingly, as well as flickering the lights and playing their respective soundtrack.

Just so you know, this is all covered on the Warframe Wikia pages as well. http://warframe.wikia.com/wiki/Harvester http://warframe.wikia.com/wiki/Stalker

The Stalker is NOT a Tenno.

His armor (Warframe) may make him look like a Tenno, but he has no operator power and there are no indications in the current story that he was present during the event that created the Tenno.

Second Dream Spoilers:

It is revealed during the events of The Second Dream that the Stalker was quite unaware of the true nature of the Tenno. Although the Tenno themselves are unaware of their own nature, it's evident that the Stalker is not part of them, for his attempted actions during the second dream would have led to his own demise. Beyond this, the codex indicates he was present (as a "low guardian") when the Tenno turned on the Orokin Empire.

More Spoilers:

Further story updates have shown that Warframes are offshoots of infested technology (the Helmith virus) that were originally controlled by something similar to sentient intelligence (an Orokin creation) to be sent after the Sentients. However, these version were easily subverted by the Sentients and proved ineffective. The Warframes were useless to the Orokin until the Tenno were found to pilot/control them. Ballas used a custom version of the virus (with modifications) to destroy/mutate a rival and control him (Excalibur Umbra). It's likely that the Stalker is the result of someone who had the ability to modify the virus and access to it to create a one off "warframe" (which is not truly a frame since the original host is likely still inside).

In Conclusion:

The key answer to this question lies in the fact that Tenno ARE NOT Warframes, they are the operator controlling the Warframe. The Stalker has no such operator.