This problem has occurred to me multiple times now. While me and my friend were playing Minecraft multiplayer (local), we had started a few raids. It was in no cheats survival so we killed the Captian and went to a natural village each time. When this first happened, there were no mobs at all. The horn seemed to be coming from not to deep underground but literally nothing was found even after intense searching. That village and raid was abandoned. A while later we had started a second raid at a different village. After the raid started, nothing was found again. The raid Bar did go down randomly this time but there were no attacking golems and we had done nothing. After lighting off about 3 tnt underground where we thought the horn was, the bar went down to 1 (but not during the blasts). There was only one village golem and it was staring at the ground doing nothing the whole time.

Mentionable things:
-In both raids we found one village golem looking at the ground (we searched where they were looking)
-The raid bar could be seen by every player no matter the position in the world
-The raid bar remained on our screens after leaving the village
-The horn came from within the village