Stress caused by environmental conditions can make orchids (and any indoor plant) more vulnerable to attack by fungi, bacteria, or insect/mite pests. Factors involved could be temperature changes and range, low humidity, and poor air circulation. Unfortunately, making a diagnosis from symptom appearance alone can be challenging, as there can be a lot of overlap between various fungal and bacterial pathogens. We do not have an orchid specialist and our plant pathologist isn't familiar with orchid diseases (we receive very few inquiries about them), but our educated guess based on symptoms is Pseudocercospora, a commonplace type of fungus.

Are the plant's leaves misted regularly? If so, we suggest you stop misting and use a room humidifier instead (if needed to boost humidity around the plant), since misting foliage both doesn't raise humidity enough or supply enough moisture to its roots, and increases the plant's vulnerability to infection. (Fungal and bacterial spores have an easier time infecting leaf tissue when leaf surfaces are wet.)

Aside from the leaf spotting, the plant appears to be in good condition, in that its foliage is of an even size and the potting mix doesn't look decomposed or too aged. If the plant is losing lower leaves (by itself, this is normal as those leaves age) more quickly than it used to, and if it is not being fertilized during the growing season (spring, summer, and early autumn), it might benefit from list doses of fertilizer (as recommended for orchids on the package label). Don't overdo it in that case, since orchid nutrient needs are not as robust as those of other indoor plants and over-fertilizing will require more frequent repotting with fresh mix to avoid damaging roots, but sometimes a deficiency in certain nutrients can speed-up the natural shedding process for its oldest leaves. This is likely unrelated to the leaf spotting issue, but we mention it because you referred to a red-colored older leaf. Since we don't see a reddened leaf in the photo, we presume it already fell off on its own.

Removal of heavily-affected individual leaves might slow the spread of a current infection or prevent it from reaching other leaves, but we can't be certain since we can't concretely diagnose it. There will be a trade-off in reducing infectious potential and removing leaf tissue that might still be able to photosynthesize to fuel plant growth and recovery; plus, cuts into live tissue still bear a risk of infection themselves, even if it's from a different pathogen.

If you attempt to treat the plant, you'd need to find a fungicide (one containing copper as the active ingredient will be one of the only options for a spray that also affects bacteria) labeled for use on indoor plants; most product are not, in which case you may need to wait until the weather is milder in spring to move plants outside (in the shade) for treatment before brining them back in once spray residues are dry. If the product label does not include orchids (or Phalaenopsis) on the list of plants its intended to treat, you may need to test it on a single leaf first and wait several days to make sure it won't produce a phytotoxic reaction before treating the entire plant. Phytotoxicity is plant tissue damage from a chemical exposure, and it can occur even with pesticides used properly in some cases. If you do try a pesticide, bear in mind that it cannot cure existing disease and affected tissues won't recover or revert to a more normal appearance, even if the treatment works to halt an infection's spread. Fungicides work by helping to prevent infections from beginning in healthy growth, and they often need more than one reapplication, the timing of which can vary from product to product, so always consult the label for details.

For future acquisitions, make sure the plant's foliage shows no signs of disease or discoloration, since eradication of an infection can be difficult. (Leaves on some orchids can take on a reddish or purplish hue if they are receiving very bright light, but that will look different compared to infections that tend to develop localized darkening, yellowing, or soft spots.) To be fair, sometimes a plant can be harboring an infection that hasn't yet produced visible symptoms, but any plant with obvious leaf spots or other damage should ideally be avoided.

Miri