I would agree that a message to Sirus would be in order. Sommers suggested we send text saying we are willing to extend the SAP, I am curious if it would be better to say we would like to continue it. That way the onus is on Sirius to turn the agreement down which would give away intentions.
I would not sink the caravel with privateers as it would be easy to suspect us. I would however think it is ok to acknowledge to Sirius that we see their sentry net and to disclose that we have similar ships patrolling the waters. Saying something along the lines of "secure borders makes for stronger allies" would make sense. We may want to consider a secondary line of privateers though should Sirius attempt to send a caravel past our sentries. Again it would be ok to disclose this and to inform them of the consequences of attempting to run the blockade. We can assume they will do the same.
Directly asking them to extend the SAP puts the onus on them to give a yes / no answer. Saying we are "willing" to extend it means they have a lot of flexibility to respond in any manner they please which they could use to better hide intentions. It is a small thing but I would rather use language that forces them to make a commitment.
I logged in to look around and I was greeted by this screen...Maybe a good strategy is to ask to extend existing treaty and then if they say yes open the discussion to new terms.
Spoiler :

Then I realized that agreeing to the trade would force us into a 10 turn peace treaty with them, which would last untill 980AD, 1 turn before the SAP treaty expires (580AD + (10x40yrs per turn)=980AD). So this seemed to me to be another indication that they are worried that we plan to invade them before the treaty expires or right after.
So I though just rejecting the trade would set off false red flags that we in-fact planning invasion. To avoid this, I put a long message in the in-game diplo window. I meant to save a screenie but I somehow exited before I could finish. I hope the parts I wrote before were sent, but I sent a folloup message to be sure. This is the gist of what I said as close as I can replicate:
I think most of the message went through, except that last paragraph, which I re-sent right after, along with a proposal for them to just gift us Marble, along with a proposal fo them to trade us Marble for Sheep, which I dont think they have from us already... or maybe they do... anyway, that's what I did.I just logged in to look around and saw this trade proposal. I have no knowledge of it and I just want to look around so I am rejecting it only for that reason. If this was already planned out and agreed to with another member of the team, I apologize.
However, it seems odd that you would ask for 1GPT for Marble. Why not just gift it to us if you are not using it? The only thing I can think of, is that the 1GPT is a way of guaranteeing a NAP for the next 10 turns. That is not necessary since we already have a treaty covering the next 10 turns.
It seems that maybe the real issue is that we need to have a more formal discussion about the treaty. Since it seems you are thinking the same thing, I will just send you a repeat of this message through regular diplo channels.
Just to be clear, me rejecting this trade should not be taken as a sign that AMAZON is planning to invade Sirius. We are planning no such thing and are eager to discuss extending our treaty with you.

I did not want to destroy our bargaining power by accepting the trade without discussing it, as now I am even more certain that their entire army was destroyed by Mavs at Injiya. That 1GPT trick seems a little bit on the nervous side...
I will send the above message to them via PM and gmail, along with a proposal to extend the SAP, along the lines we discussed earlier. I will post the whole thing here when it is sent