Natsu would be really angry at whoever or whatever caused him to lose his scarf. He might go on a bit of a rampage to find it. And he'd be super determined, like when he goes after his enemies. Because for him, that scarf is more than just a piece of cloth. It represents his connection to Igneel, so he'd stop at nothing to get it back.
At first, he might be in shock. He's so used to having his scarf with him all the time. Then, he'd start thinking about how to get it back. He could enlist the help of his friends from Fairy Tail. They'd all know how important the scarf is to him, so they'd probably be just as eager to find it. Natsu would be constantly worried about it until it was back in his possession, and during the search, he might be a bit more on edge than usual.
Natsu would probably be very angry at first. He loves Fairy Tail like a family, so being kicked out would feel like a huge betrayal. He might go on a rampage, shouting and arguing with the guild members who made the decision.
Natsu would be heartbroken. Fairy Tail is like his family. He'd probably go on a solo journey to clear his name and prove his loyalty. Erza would be stoic but deeply hurt. She'd start an investigation within the guild, using her influence and intelligence to uncover the truth. She might also isolate herself a bit while she figures things out.
Gray would be shocked at first. He and Natsu have a long - standing rivalry - but also a deep friendship. He might initially be angry that Natsu would just leave like that, but then he would realize how much he'll miss him. He might also feel a sense of loss for the guild as a whole, since Natsu is such an important part of it.
The guild would likely be ecstatic. Gray would probably make some snarky comment about Natsu finally growing up, but deep down he'd be happy for his friend. Erza would be all for it and might start planning the wedding immediately, giving orders to everyone in the guild to help out.
Gray would be angry and sad at the same time. He was Natsu's rival but also his friend. He might lash out at first, blaming the situation or other people. But then he would turn his anger into a determination to find out what really happened to Natsu and seek justice if there was foul play involved. And he would also be there for Lucy and the other guild members, trying to keep them strong.
Their friends would likely be very excited. Lucy would probably be overjoyed and offer to help with things like baby clothes shopping. Gray might act a bit gruff on the outside but secretly be happy for them. Erza would be very protective, making sure that Lisanna has everything she needs and that the guild is a safe environment for the baby. Happy, Natsu's Exceed friend, would be constantly around, making cute comments about the baby and offering his own unique brand of support.
He might initially be shocked and confused. Natsu has always been so loyal to Fairy Tail, so this would be a huge blow to him.
He might initially be in disbelief. Natsu is used to the loyalty among his Fairy Tail comrades, so betrayal would be a shock to him.
Perhaps there's a magical mishap. A spell gone wrong could cause the scarf to disappear. It might be a spell that was aimed at Natsu but instead of harming him directly, it targets his most precious item, which is his scarf. This could lead to a whole adventure where Natsu tries to get his scarf back, facing various challenges along the way.
Most of the Fairy Tail members would not take it well. Happy, Natsu's closest friend, would be extremely confused and sad. He'd follow Natsu around, trying to get him to come back to the guild. Makarov, the guild master, would see it as a huge threat to the guild's safety. He might send some of the stronger members to keep an eye on Natsu and Zeref. And the younger members like Wendy would look up to Natsu so much that they'd be very disappointed and scared for him, thinking that he's making a big mistake by joining Zeref.