HardwareSerial/SoftwareSerial::read() return a byte where the functions are defined to return an int. The implicit conversion from "unsigned char/uint8_t" to int returns 0xFFAA (= -86) for the received byte 0xAA.
The function Stream::findMulti() then tests the integer returned by Stream::timedRead() with "c < 0" instead of "c == -1" to distinguish between a received byte and -1 for still nothing available on the stream.
Therefore the function Stream.find() comes back with -1 after timeout, although read() received the byte 0xAA.
It took a lot of time to find out the reason why I didn't found the start byte 0xAA on the stream from my MP3 module response.