In the original Japanese version, Robo and Mobo were actually thieves, not just hired to test security systems or recover evidence.

Their character bio in Sonic & Sega All-Stars Racing gives their original backstory.
Contributed by ST3
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In the Sega Genesis version, Opa-Opa, of SEGA's Fantasy Zone series, makes a cameo appearance as a painting in Level 9.
Contributed by NerdyBoutKirby