Why does Nexus 5 HCE respond to FELICA select? I have an NFC reader constantly polling for tags present. I also have a Nexus 5 with a HostApduService activated. When repeatedly tapping the Nexus 5 on the reader, the reader reports a FELICA (JIS: X6319-4) tag found in 9 out of 10 cases. About 1 in 10 it reports an ISO14443-3A device.

The latter is the expected behavior and the one described for Android HCE. I cannot find mention of this FELICA support anywhere. A OnePlus One only shows up as ISO14443-3A, for example.

Why does a Nexus 5 HCE show up as FELICA? And is there a way to disable this behavior?

[EDIT]: I have two Nexus 5 phones here, acting differently. This one acts like FELICA: Android 5.1, Build LMY47I. The other does not: Android 5.1.1, Build LMY48M
Besides HCE mode, an Android device typically also operates in NFC peer-to-peer mode. Passive peer-to-peer mode (ISO/IEC 18092) is based on the anti-collision and activation protocols from ISO/IEC 14443 (for 106 kbps) and from JIS X 6319-4 (for 212 kbps and 424 kbps). Hence, devices in peer-to-peer mode may be detected as JIS X 6319-4 cards as well as ISO/IEC 14443-3 cards by contactless smartcard readers.

How likely it is to (accidentally) detect an Android phone as "FeliCa" or to (properly) detect it as ISO/IEC 14443 depends on the implementation of the NFC polling/listening loop that the phone uses to cycle between the different active and passive NFC (and NFC-compatible) protocols.