Did I stumble upon a cell hidden network or is my device being attacked?

I strongly suspect those four characters are an "A", a "T", an "&" (the Sun symbol) and a "T" again. Possibly the UTF8 for 4F10, 5418, E298BC and 5418.

Actually, as @Matt observed, the little-endian UTF16 sequence 4100 5400 2600 5400 ("AT&T"), if interpreted as big-endian, becomes 䄀吀☀吀 which closely matches the strange logo that appeared in the cellphone.

Possibly whatever packet contained the carrier "logo" got corrupted, CRC notwithstanding, and was interpreted as a foreign character set.

Update: Two users added comments (unfortunately posting them as answers, which got them deleted) confirming that this happened to them; in one case on September 29th and 30th, "several times [...] traveling through North Dakota, Minnesota, Wisconsin, Illinois and Indiana... usually when AT&T signals were weak."