I think I would use a flash grenade as a way to use weapons instead of stealth to set up an ambush. Throw flash grenades with weapons, defend with alertness, and if the attacker succeeds, it's an ambush, meaning the attacker gets to act first, and the defender's defense skills are treated as 0 for the first exchange.
That's not a declaration though. With a declaration you can add details to the story. So in your example, the group might be going in, and when they spot the enemy around the corner, the ex-military guy says it would be best to take them out with flash grenades. Since nobody explicitly has some, he could declare that he has some. Given his background and the people he knows, it makes sense, so the GM sets the difficulty only at 2. He rolls his contacts to declare he just got a pack from his arms dealer and he brought some with them.
So the flash grenades were never there at first, but he declared them to be there, so he had them with him, they just didn't matter until now.
Equally, a catburglar could declare a ventilation shaft just above the guards, so he climbs through and takes them out from above. Or any number of details that could be used to your advantage.
Since a declaration doesn't take an action, you shouldn't overuse them. Not every detail has to be an aspect and give a bonus, sometimes thing are just there and that's that. If a declaration seems too far fetched, the GM can also ask the player to pay a Fate point for it.