Would one be allowed to use an air purifier with an LED smell indicator sensor on Shabbat if bringing in odors near the purifier with outside food or other odorous things could or most surely would trigger an LED sensor to go off? The resulting activation of the LED lights would be undesired and the trigger would technically be an indirect delayed activation unlike walking directly in front of a motion LED sensor. The benefit of the purifier would be to control allergy symptoms whereas without it allergies would be worse. Also there is no way to disable the LED lights.
Rav Nachum Rabinovitch, shlit"a, (who holds electricity is only, at worst, an issue of uvda d'chol [weekday activity] has ruled allowing the use of hotel key cards even when they trigger an LED (see here). Your scenario would seem to be even less of an issue. (I believe he has also accordingly allowed the use of baby monitors even where an LED indicates crying.)
Similarly, according to those such as the Chazon Ish, z"l, who hold that electricity would only be an issue of causing a useless item to become a useful tool (mekeh b'patish-finishing a utensil) or structure (boneh-building), this scenario would seem to be fine since the device is operational before the LED is triggered.
The only issue might be according to those, such as Rav Chaim Ozer, z"l, who hold that electricity is a corollary of lighting/extinguishing a fire. Notably, while an incandescent bulb, which generates both heat and light, is arguably included within the Talmudic example of a gacheleth shel matecheth (burning metal shard; see here and here), an LED does not generally involve significant heat, which perhaps suggests it should not be classes as esh. (This argument might require assuming that the amud esh [pillar of fire] in the desert did generate heat - which may be a reasonable interpretation regardless considering the desert does typically get cold at night.) In any case, I am under the impression that Rav Shlomo Zalman Auerbach, z"l, who originally held like Rav Chaim Ozer, z"l, retracted after in-depth consultations with scientific experts in the field, though he was unwilling to reverse the popular stringent view of electricity in general.