Small Aircraft; Low Wing; 1 Eng; Retractable Gear
Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Multiengine; Flight Crew Instrument
Training / Qualification; Communication Breakdown
Deviation / Discrepancy - Procedural Clearance; Ground Incursion Taxiway
Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance
Human Factors
Upon Landing and clearing the runway XXL at ZZZ; I believed that tower stated 'Next left; remain this frequency; taxi to park'.  Unfortunately tower actually said 'Next left; contact ground point nine' as I played back from the avionics playback function after tower strangely issued me a taxi clearance while I was already taxiing. I read back the (what I thought was a re-issued clearance) and began to question if I heard the initial runway clearing call correctly. After utilizing the playback feature; I realized I taxied without a clearance.  With the reduced flight activity of the COVID-19 pandemic; I need to slow down; take more time; and be extra cautious. This is amplified by my expectation bias from the past few flights as tower has given me the 'Monitor and taxi to park' clearance. Clearance; ground; and tower have all been combined lately due to reduced activity.  In many cases; clearance has had us taxi with tower frequency; and such.  All the more reason to be extra cautious. Take additional time; and listen carefully as we are all in very non-standard operations.
Pilot reported a taxiway incursion partially due to expectation bias.