The preamble is intended for synchronization, the last two consequtive ones indicate the start of the data.

My circuit doesn't drop any frames, it simply converts serial Ethernet signal to SPI. FCS and MAC checking is done in software after receiving the whole frame.

You absolutely can get frames not addressed to you. Wireshark lets you do that. I debugged my adapter this way: I connected it to a dumb switch together with my computer and ran Wireshark on the computer which captured all frames regardless of their intended destination. On Linux you can even disable FCS checking and receive all rubbish from the wire.