A method comprising: receiving, by a computer (108, 302, 508, 708) and from an access device (104, 204, 506, 706) or a communication device (102, 202, 401, 502, 702), an authorization request message in a current transaction; obtaining, by the computer, current network data based on a current set of wireless networks that are within a wireless network detection range of the access device or the communication device interacting with the access device during the current transaction; the method being characterised by: determining, by the computer, a difference between the current network data and previous network data, wherein the previous network data is based on one or more previous sets of wireless networks that were within the wireless network detection range of the access device or the communication device during one or more previous transactions, wherein determining the difference between the current network data and the previous network data comprises determining one or more unrecognised wireless networks in the current set of wireless networks, the one or more unrecognised wireless networks not being in any of the one or more previous sets of wireless networks; determining, by the computer, that the difference exceeds a threshold corresponding to a number of unrecognised wireless networks; and responsive to determining that the difference exceeds the threshold, initiating, by the computer, one or more actions associated with the current transaction. receiving, by a computer (108, 302, 508, 708) and from an access device (104, 204, 506, 706) or a communication device (102, 202, 401, 502, 702), an authorization request message in a current transaction; obtaining, by the computer, current network data based on a current set of wireless networks that are within a wireless network detection range of the access device or the communication device interacting with the access device during the current transaction; the method being characterised by: determining, by the computer, a difference between the current network data and previous network data, wherein the previous network data is based on one or more previous sets of wireless networks that were within the wireless network detection range of the access device or the communication device during one or more previous transactions, wherein determining the difference between the current network data and the previous network data comprises determining one or more unrecognised wireless networks in the current set of wireless networks, the one or more unrecognised wireless networks not being in any of the one or more previous sets of wireless networks; determining, by the computer, that the difference exceeds a threshold corresponding to a number of unrecognised wireless networks; and responsive to determining that the difference exceeds the threshold, initiating, by the computer, one or more actions associated with the current transaction. determining, by the computer, a difference between the current network data and previous network data, wherein the previous network data is based on one or more previous sets of wireless networks that were within the wireless network detection range of the access device or the communication device during one or more previous transactions, wherein determining the difference between the current network data and the previous network data comprises determining one or more unrecognised wireless networks in the current set of wireless networks, the one or more unrecognised wireless networks not being in any of the one or more previous sets of wireless networks; determining, by the computer, that the difference exceeds a threshold corresponding to a number of unrecognised wireless networks; and responsive to determining that the difference exceeds the threshold, initiating, by the computer, one or more actions associated with the current transaction.