The poem catches a small moment of confusion. The speaker hears a noise and assumes someone nearby is setting an alarm. Then an ambulance siren seems to swell, and the sound turns out to be, "after all," the crossing signal. What the speaker first took for one thing was something else, and the mistake keeps shifting: alarm, ambulance, signal.
The last two lines carry the title's link to Escher. "Table and street / were on one surface" suggests that the indoor, domestic space and the outdoor, public one have merged, so a sound from the street feels as if it belongs in the room. The poem seems to treat this as a quiet, slightly dizzying blur of inside and outside.
The speaker sounds calm but unsettled, more puzzled than frightened. The short, hesitant lines mimic the way perception gets corrected midway. The mood is a mild disorientation, as though the ordinary world briefly lost its edges.