Speak with the dead:Find out how the hell the assassins got by creatures with Scent. Scent is auto detect within range so stealth cannot have any bearing.
Good point, and something to take into account. Your DM may be accidentally cheating. If he did not realize that the wargs had Scent and what that would mean for his assassin, he may have erroneously ruled that the assassin made his stealth roll and walked past the dogs, thinking it was that simple.
Your problem will be helping your DM save face to acknowledge the mistake. He likely won't want to recon it. And that'll mean he'll just supply a reason Scent didn't work. You might get him to back off on these attacks since he made the mistake of mishandling them in the first place.
A reasonable statement might be: "We've looked at these assassins and it doesn't appear they would have had the ability to bypass the Wargs Scent ability. If this is true, then you may have mishandled these assassination attempts. If so, I think it would be fair that you back off on these attempts for awhile as counterbalance to the mistake. If you do indeed have all this accounted for, then carry on as normal."
A statement like that leaves room for the GM to correct himself. It also shouldn't blatantly tell him what to do. but it does draw a line that the GM can't just make stuff happen and bypass reasonable precautions.
That said, take more precautions.
Somebody knows your itenerary or is tracking you. Find out who, and/or block it. That means sleeping in a different tent, giving a misleading travel plan, etc. Even the Rope trick where it goes into an alternate space. Put the rope on a tent pole, and folks may not even realize what it is, simply seeing you are not in your tent.
You've had multiple breaches. Change out the security. Or leave the existing and secretely add more that the first doesn't know about.
Run trip-wires to pull on bells or rattle pots and pans around the perimeter.
If you don't use the Rope Trick, put a portable hole on the floor of your tent. Basically, enter the tent, climb down into the hole, and sleep there. That way, if they do a drive-by shooting, you will be beneath the line of fire and thus not get hurt.
Compartmentalize information about your plans, security and travel. Follow a strict Need To Know policy. This will minimize how much outsides can glean about where you will be and your security arrangements.
Coat your tent walls with something that blocks scrying/listening devices. In the real world, building a Farraday Cage inside the walls of a room in your house would block cell phone, wifi, and other electronic broadcasts. Same concept in D&D. Usually, that material is Lead. which is harder to make a portable tent material out of.
You might also want to figure how how to cleanse/replace any items that might be used to track you. Peruse the PH for spells that track or listen, as that might be where your security leak is.